This invention relates to the control of heat flux in continuous metal casters, particularly (although not exclusively) those used for the continuous casting of aluminum and aluminum alloys. More particularly, the invention relates to a process of casting a molten metal to form a cast metal strip ingot while exerting control over the rate of withdrawal of heat from the cast metal to avoid surface defects and distortions of the casting cavity. The invention also relates to apparatus used in the process.
Continuous casters, such as twin belt casters and recirculating block casters, are commonly used for producing strip ingots (continuous metal strips) from molten metals, particularly aluminum alloys. In casters of this kind, a casting cavity is formed between continuously moving casting surfaces and molten metal is introduced into the casting cavity on a continuous basis. Heat is withdrawn from the metal via the casting surfaces and the metal solidifies in the form of a strip ingot that is continuously withdrawn from the casting cavity by the moving casting surfaces. The heat flux (or heat extracted from the solidifying metal) must be carefully controlled to achieve cast strip ingot of good surface quality and to avoid distortion of the casting cavity. Different metals (e.g. aluminum alloys) require different levels of heat flux for proper casting on a continuous basis, so it is important to be able to control the casting apparatus to provide the required levels of heat flux for a particular metal being cast.
The primary heat flux control is usually achieved by applying cooling water to the casting surfaces. In most belt casters this is done on the back face of the belt passing though the casting cavity. Other caster designs apply cooling water at positions remote from the casting cavity. However, the heat flux is often adjusted more precisely by additional means. For example, belt casters have been provided with porous ceramic coatings over the metal belts. Such coatings may optionally be partially or completely filled with a high conductivity inert gas, such as helium, to provide further refinement. In such cases, the expense of maintaining a consistent ceramic coating and the cost of the inert gas have made such procedures economically unattractive.
It is also known to apply a layer of a non-volatile liquid, e.g. an oil, to the casting surfaces before they come into contact with the molten metal. This layer is often referred to as xe2x80x9cbelt dressingxe2x80x9d or as a xe2x80x9cparting layerxe2x80x9d. The thickness of the layer can be varied to provide for control of heat flux to the underlying casting surfaces. However, the use of such oils may adversely affect the surface quality of the cast strip ingot (particularly ingots made from aluminum alloys containing high levels of magnesium), and may give rise to environmental issues, particularly when excessive applications are required in order to achieve the desired degree of heat flux control.
An example of a continuous casting apparatus requiring heat flux control is described in U.S. Pat. No. 4,593,742 which issued on Jun. 10, 1986 to Hazelett et al., and was assigned to Hazelett Strip-Casting Corporation. The apparatus of the patent is a twin belt caster employing a flexible nozzle for introducing molten metal into the casting cavity formed between the belts. Heat flux is withdrawn through the casting belts by means of a high velocity moving layer of liquid coolant traveling along the reverse surfaces of the belts. In this patent, mention is made of the supply of a non-reactive (inert) protective gas to the inlet of the casting cavity to protect the molten metal from chemical attack.
U.S. Pat. No. 3,630,266, which issued on Dec. 28, 1971 to Leonard Watts, and was assigned to Technicon Corporation, also discloses a continuous caster having a casting nozzle introducing molten metal into a cooled casting cavity. In this case, a gas is supplied to the region of the cavity inlet to insulate the casting nozzle and to prevent the formation of solidified metal bridges between the nozzle and the cavity.
An object of the invention is to facilitate the control of heat flux in continuous casting apparatus used for producing metal strip ingots from molten metals, particularly aluminum and aluminum alloys.
Another object of the invention is to enable the production of high surface quality metal strip ingots from continuous casting apparatus under changing operational conditions.
In the present invention, mention is made of the xe2x80x9cregion of the meniscusxe2x80x9d. This is the open region (i.e. not containing molten metal) within the casting apparatus where the molten metal first engages a casting surface (forming a meniscus) and is therefore adjacent to the meniscus and is generally in gaseous communication with the exterior of the casting apparatus.
The present invention uses a controlled source of water vapour (steam) to create a stream of gas (usually air) of known and easily controllable humidity, which is used to flood the area of the caster in the region of the meniscus. It has been found that this produces an effect on the heat flux that is much larger than would be expected based on the change in the thermal conductivity of the gas brought about by the addition of the moisture. This can be used as a convenient and relatively inexpensive way of avoiding thermal distortion by controlling heat flux from the caster in way that, in particular, may be used with existing casting equipment with minor modification.
In one preferred aspect, the present invention provides a process of casting a molten metal to form a cast metal strip ingot, in which good heat flux control may be provided. The process continuously supplies molten metal to a casting cavity formed between a pair of moving continuous casting surfaces that withdraw heat from the molten metal to cause metal solidification, and continuously withdraws a resulting cast strip ingot from the casting cavity. The molten metal at an inlet of the casting cavity forms at least one meniscus at a position where the molten metal first contacts the casting surfaces. The invention involves supplying a gas containing water vapour substantially without liquid water to the inlet of the casting cavity in the region of the meniscus (a region containing the meniscus(es)) to control the heat withdrawal by the casting. Preferably there is sufficient space between the casting surface and the solidifying metal strip such that gas can penetrate the space during casting.
As an example of typical equipment and processor to which the present invention may be applied, there may be mentioned Sivilotti U.S. Pat. No. 4,061,177 incorporated herein by reference.
The heat withdrawal may be controlled to a single value by measuring the heat flux or temperature at some point along the casting cavity and comparing the measurement to a target parameter, or may be controlled to a predetermined function along the casting cavity by means of multiple heat flux or temperature measurements. Temperature measurements may include slab temperature measurements, including measurements at the exit of the casting cavity or temperature measurements at points behind the casting surface within the casting cavity. Heat fluxes, for example, may be determined by measuring the temperature increase of the coolant used to cool the casting surface in one or more locations and the flow rate of that coolant.
The gas containing water vapour may be obtained in a number of ways. It may be created, for example, by mixing a dry gas and steam externally of the region of the meniscus or within the region of the meniscus. For example, the gas may be supplied by providing a porous block or similar device adjacent to the region of the meniscus so that the porous block becomes heated by the molten metal, injecting liquid water into the interior of the porous block so that the liquid water is vapourized within the heated porous block and thereby forms a mixture of gas containing water vapour in the regions of the meniscus. However, it is particularly preferred that the gas containing water vapour be provided as a premixed mixture from an external apparatus. This gas containing water vapour may be formed by mixing a dry gas, such as air, with water vapour. Other dry gases that may be used include nitrogen or an inert gas such as helium or argon. This mixing operation may be carried out at a temperature above the desired final dewpoint of the mixture, then the final dewpoint established by passing the gas containing water vapour through a heat exchanger at the desired dewpoint temperature so as to remove excess water vapour from the mixture. However, it is preferred that such a mixing operation be carried out by controlling the relative amounts of water vapour and dry air entering a mixing chamber in response to a measurement on the resulting stream of gas containing water vapour.
The exact dewpoint of the gas required to control the heat withdrawal by the casting surfaces to a predetermined value may vary and is dependent on a number of parameters including the ambient conditions surrounding the caster (since the casting cavity is not specifically sealed from the outside conditions), and the quantity and nature of any belt dressing or parting layer that might be applied. Generally delivery of a gas having a dewpoint between xe2x88x9260xc2x0 C. and +70xc2x0 C. will ensure control of heat withdrawal under all conditions. The gas mixture will of course have to be heated to above the dewpoint to prevent premature loss of moisture. For that reason, an upper limit of +30xc2x0 C. will be more generally preferred, and in general a dewpoint of greater than xe2x88x9225xc2x0 C. will also meet most requirements.
The casting surfaces are preferably textured or treated to create microscopic passages to improve the penetration of gas into the space between the casting elements and the solidifying ingot. For example, the casting elements may be shot blasted to roughen them or a texture may be applied by knurling techniques.
When aluminum or aluminum alloys are cast in accordance with this method the cast slab surface is substantially free of oxides and can be rolled to final thickness without cleaning to remove oxides.
According to another aspect of the invention, there is provided an apparatus for casting a molten metal to form a cast strip ingot, comprising: a pair of moving continuous casting elements arranged to form a casting cavity between opposed casting surfaces of said casting elements; a nozzle for continuously introducing molten metal into said casting cavity and forming a meniscus where said molten metal first contacts said casting surfaces; and equipment for producing a gas containing water vapour substantially without liquid water and for delivering said gas to a region of said meniscus.
In the apparatus, the stated equipment preferably includes a mixer that mixes the dry gas and water vapour externally to the region of the meniscus. Preferably, the equipment includes a mixer for mixing dry gas and steam to produce said gas containing water vapour, and the equipment also preferably contains a detector for measuring the temperature and the water content of the gas containing water vapour, and a calculator for calculating the dewpoint of the gas containing water vapour from said measured temperature and water content. The equipment also preferably contains controls for adjusting amounts of dry gas and steam mixed by the mixer according to signals produced by the calculator to produce said gas containing water vapour having a predetermined dewpoint.
Another aspect of the invention relates to an apparatus and method used for producing a supply of moist gas having a predetermined dewpoint. The dewpoint is typically in the range of about xe2x88x9260xc2x0 C. to +25xc2x0 C. The apparatus has a mixing vessel for receiving and mixing steam and a dry gas, a steam generator for generating the steam, and a supply of the dry gas. The apparatus includes a delivery conduit for delivering moist gas from the mixing vessel to the casting apparatus (or other apparatus). The delivery conduit includes a detector device for determining the dewpoint of the moist gas. Such a detector device preferably includes a detector for detecting the moisture content of moist gas passing through the delivery conduit, a detector for detecting the temperature of moist gas passing through the delivery conduit and a calculator for calculating the dewpoint of the moist gas passing through the delivery conduit. A controller is also provided for adjusting the supply to the mixing vessel of one or both of the dry gas and the steam to cause the moist air to exhibit a predetermined dewpoint.
It has been found that the change in the gas used to flood the region of the meniscus (the xe2x80x9cflooding gasxe2x80x9d) from essentially dry gas (air) to gas having a dewpoint of say 15xc2x0 C. produces a change in heat flux of 3% to 4%. This is at least a 10 times greater change than would be predicted on the basis of thermal conductivity alone. Furthermore, for casters that use oil as a parting layer applied to the casting surfaces, the changes in heat flux actually produced by the invention may be equivalent to increasing the amount of oil feed by 20% or more, which is a substantial saving.
This invention is particularly preferred for use in continuous strip casters having elongated casting cavities. Such casters include block and twin belt casters. In such continuous strip casters, the casting surfaces often have to absorb a high heat flux in the region of the meniscus, and this heat flux generally decreases further along the casting cavity. It has been found that the present invention reduces the initial high heat flux, by broadening and lowering the heat flux peak resulting from molten metal initially contacting the casting surface, or by reducing the initial heat flux and increasing the heat flux further along the cavity, and this has the effect of reducing thermal stresses on the casting surface.
It is particularly preferred to use block or twin belt casters with a liquid parting layer, e.g. an organic material such as oil or mixtures of solids in such liquid carriers. The parting layer is preferably applied to the casting surface before it contacts the molten metal, and may be removed after the casting cavity. Systems for application and removal of such parting layers are described, for example, in U.S. Pat. No. 5,636,681 (Sivilotti et al.) incorporated herein by reference.
In cases where the initial heat flux is very high, thermally induced distortions may occur in the casting surface. It has been found that the present invention can lower this initial high heat flux and distribute the flux more uniformly along the casting cavity, thus reducing the potential for distortion of the casting surface.
It is believed that in the area where the meniscus contacts the casting surface, and for a considerable distance beyond that point, there is a microscopic gap between the solidifying metal and the casting surface, which communicates with the region of the meniscus. Gas and water vapour provided to the region of the meniscus infiltrates this area via the microscopic communicating gap and the effect of the gas and water vapour on the heat flux combines with the effect of the parting layer over a considerable distance (i.e. well beyond the meniscus), thereby having a substantial effect on the distribution of heat flux.
The microscopic gap is believed to be a result of the roughness of the surface (which may be enhanced by treatments such as shot blasting or knurling the surface) and the shrinkage on freezing of the metal. In this gap, the liquid parting layer starts to vapourize and form a vapour layer which modifies the heat transfer between the metal and the casting surface, and hence the cooling rate of the metal. The presence of water vapour in this gap further modifies the heat transfer in this gap.
The gas containing water vapour is supplied at a rate that causes continuous flooding of the region containing the meniscus to exclude ambient atmospheric air therefrom. However, the gas flow rate or pressure must not be so great as to deflect or displace the meniscus during operation.
The casting surfaces are preferably cooled by the application of a coolant (generally water) to the reverse side of the casting surface in the region where the casting surface and metal cast strip are in proximity. Coolant is preferably applied from a point ahead of the region of the meniscus to a point beyond which the metal cast slab is fully solidified. Sufficient coolant is applied to the reverse of the casting surfaces in advance of the region of the meniscus to ensure that the surface temperature of the casting surface immediately before contacting molten metal in the casting cavity is less than 100xc2x0 C., and preferably less than 50xc2x0 C. Thus it is preferred that the casting belts not be preheated.
A variety of different metals may be cast according to the invention, particularly those with relatively low melting points. However, the invention is of particular value for the casting of aluminum and alloys thereof. It is, in fact, surprising, given the reactivity of aluminum in the presence of water vapour, that the invention can be used for aluminum and aluminum alloys.